The present invention includes methods and apparatus for making connections to an integrated circuit chip. The Via-Less Two-Metal Tape-Automated Bonding System provides a cost-effective and reliable device that overcomes the signal degradation that is encountered when single-metal TAB technology is employed in high speed digital and high frequency analog electronic systems.
Each year integrated circuits become more powerful and capable of storing more information. One of greatest challenges confronting designers in the electronics industry is finding more efficient and reliable methods to access the complex circuitry that resides within the chip package. A large rectangular IC chip having sides less than one half inch in length may have as many as five hundred or more leads extending from it. Each of these conductors must be physically bonded or coupled to external devices.
The basic methods of mass-producing connections to computer chips utilize a technique called "tape-automated bonding." This fabrication procedure is commonly referred to by the acronym "TAB" and is well known to persons ordinarily skilled in the electronics packaging art. A continuous insulative tape which is similar to photographic film provides a planar support for chips that are attached to individual sections or frames of the tape. TAB frames are generally rectangular or square sections that are arranged side-by-side along an uncut tape. A spider-like metal pattern of conductive traces is formed on each frame. The traces radiate from the center of the frame to its four edges. A chip is attached to the center of the TAB frame, so that the leads or contacts of the chip are precisely mated with the corresponding metal traces in the central portion of the TAB frame. The resulting assembly comprising the chip, the TAB frame, and the substrate is essentially a space transformer that employs diverging radial electrical pathways to afford ready access to the integrated circuit. FIG. 1 illustrates a conventional TAB frame application 10, in which a chip 12 is bonded in the center of a TAB frame 14. The contact pads of the chip 12 are bonded to the inner leads 16 of the TAB frame 14. The inner leads 16 are connected to outer leads 18 that extend out to the periphery of the frame 14. The TAB frame 14 is fastened to a printed circuit board 20 that includes conductive traces 22 that are connected to outer leads 18. A series of through-holes 24 are used to connect different layers of the board. The device pictured in FIG. 1 is called a "single-metal tape," since it bears one conductive metal pattern on one side of the insulative carrier film.
The electrical performance of a single-metal TAB device may not always meet the requirements of high speed digital and high frequency analog systems. Each trace or lead exhibits a self-inductance, a capacitance to ground, and a multiplicity of mutual inductances and capacitances with all the other conductors in its vicinity. The parasitic self-inductance and capacitance of a TAB lead limit the frequency bandwidth of the circuit that includes that TAB lead. As a consequence, high frequency signals propagating along this circuit will be degraded, often to the extent that the signal is corrupted so badly that the data which was intended to be transmitted is misinterpreted. Another deleterious effect produced by these parasitic inductances is the unpredictable characteristic impedance that is exhibited by single-metal TAB architecture. Because the impedance levels are neither controllable nor uniform throughout such a circuit, impedance mismatches among different portions of the circuit cause unwanted signal reflections at circuit interfaces. These reflections further degrade the signal waveform. Yet another problem encountered in single-metal TAB devices is cross-talk, an interference phenomenon that results from mutual parasitic reactances between one conductive trace and its neighbors, which leads to the undesirable transmission of signals among them.
One well-known method of reducing this interference is to add a layer of metal called a ground plane on the side of the insulative film that is opposite the side that carries the signal traces. When a ground plane is added to the back side of a dielectric film that bears a pattern of conductive traces on its front side, the resulting structure is called a "two-metal TAB frame." By placing this conductive layer that is held at ground potential near the signal leads, the self-inductance of the individual signal leads as well as the mutual inductance and capacitance experienced among them can be significantly reduced. The use of the ground plane not only improves signal fidelity, but also renders the self-inductance and self-capacitance of each lead precisely predictable and controllable. Since a two-metal TAB device presents the inherent advantage of providing a known characteristic impedance that will match other sections of a circuit, the two-metal TAB is also referred to as the "controlled-impedance TAB."
This two metal layer device, however, can be exceedingly difficult and expensive to manufacture. Previous fabrication techniques incorporate the use of connections called "vias" that pass through the dielectric material to couple conductive elements formed on the opposite sides of the dielectric layer. FIG. 2 illustrates a conventional two-metal TAB frame 25 in which conductive traces 26 are formed on a dielectric film 27 that has a metal ground plane 28 fastened to the side of the dielectric layer that is opposite from the side occupied by the conductive traces 26. A ground lead 29 is connected to the ground plane 28 by metal vias 30 that penetrate through the dielectric layer 27.
Several serious problems are encountered when these vias are formed to link conductive traces to ground planes on the opposite side of the dielectric layer. The traces are spaced together as closely as possible in order to maximize the density of leads on the TAB frame. The center-to-center distance, or "pitch," across a pair of leads may measure less than five mils. As the traces become finer and finer, it becomes more and more difficult to form holes in them that can accommodate vias. Techniques for making via holes include punching, laser drilling, or chemical methods such as wet or dry etching. Each of these methods is limited by a minimum hole size that can be formed with a high degree of accuracy that produces a sufficiently high yield rate. Typically, the minimum via hole size exceeds the width of the trace or the lead that must be connected to a ground plane below it. This constraint limits the placement of via holes to locations on the TAB frame where the traces have fanned out to provide enough space to accommodate a via hole. As TAB geometry becomes finer and more miniaturized, this problem becomes more severe, since the placement of the via holes then dictates the arrangement of traces on the dielectric film. Even when via holes are made successfully and the electrical performance of TAB device is somewhat improved, the performance still does not meet many design objectives.
Unfortunately, only a very limited number of TAB tape suppliers in the world are currently capable of producing a two or multi-metal TAB device. The problem posed by the manufacture of these structures is the extremely difficult job of consistently and accurately forming the fine holes that accommodate the vias that connect the ground leads to the ground plane. Because this process is so tedious and since current yield rates are so low, the average cost of two-metal TAB frames are considerably greater than the cost of two equivalent single-metal TAB frames. The expensive two-metal TAB devices that are available have generally been found to be less reliable than their single-metal counterparts. Most of the failures that have been observed involve the vias, which are the weakest elements in conventional two-metal TAB architecture. Even though the electrical performance of a two-metal TAB is usually better than that of a one-metal device, the use of the vias poses its own performance limitations and diminishes the overall advantage of employing a conventional two-metal TAB to make connections to a chip.
The problem of providing reliable multi-metal TAB frame devices that avoid the detrimental side effects that inherently accompany the use of conductive vias that penetrate through the dielectric layer in the center of the TAB frame has presented a major challenge to designers in the automated electronic packaging field. The development of a relatively low cost, rugged, and versatile system that does not suffer from the design constraints imposed by these vias would constitute a major technological advance in the electronics business. The enhanced performance that could be achieved using such an innovative device would satisfy a long felt need within the industry and would enable electronic equipment manufactures to save substantial expenditures of time and money.